Compression release braking, or engine braking, may be employed to assist and supplement wheel brakes in slowing heavy machines, such as, on-highway trucks, construction machines, earthmoving machines, and the like. As known in the art, compression release braking converts an internal combustion engine from a power generating unit into a power consuming air compressor through selective control of various engine valves. In an embodiment, a compression release braking system actuates a cylinder exhaust valve such that compressed air from the compression stroke of the engine is released through the exhaust valve when the piston in the cylinder nears the top-dead-center position. Generally, the exhaust valve is actuated by a rocker arm that, in turn, is often operatively connected to the exhaust valve by way of a valve bridge. The rocking motion of the rocker arm presses down on the valve bridge (or directly on the valve) which in turn opens the exhaust valve, releasing the compressed air.
An automatic lash adjuster or, in most instances, an hydraulic lash adjuster (referred to hereinafter as an automatic lash adjuster) is often disposed in the rocker arm or elsewhere in the valvetrain, e.g., directly on or above the valve bridge, so as to maintain zero clearance (or lash) between the rocker arm and the valve or valve bridge during positive power generation by the engine. Examples of hydraulic lash adjusters may be found in U.S. Pat. No. 2,808,818 and European Patent Application Publication No. 0190418A1. An example of a mechanical automatic lash adjuster may be found in International Patent Application Publication No. WO2013136508A1. The teachings of these references are incorporated herein by this reference. Using a hydraulic lash adjuster as an example, the automatic lash adjuster may include a hollow, sliding plunger operated by a hydraulic fluid, such as engine oil. When the engine valve is closed, the automatic lash adjuster may be free to fill with the hydraulic fluid, expanding the automatic lash adjuster and thereby taking up lash space as it expands. When the lash adjuster is loaded, the fluid supply to the hydraulic lash adjuster may be blocked and fluid pressure within the automatic lash adjuster prevents the plunger from collapsing. In this manner, the automatic lash adjuster is able to take up any lash space between components used to actuate an engine valve.
An example of such a system 100 is schematically illustrated in FIG. 1. In particular, the system comprises a main motion source 102 used to actuate (or provide motions to) one or more engine valves 104 via a main motion load path or valve train 106. As used herein, a motion source is any component that dictates the motions to be applied to an engine valve, e.g., a cam. Conversely, a motion load path or valve train comprises any one or more components deployed between a motion source and an engine valve and used to convey motions provided by the motion source to the engine valve, e.g., tappets, rocker arms, pushrods, valve bridges, automatic lash adjusters, etc. Furthermore, as used herein, the descriptor “main” or “primary” refers to features of the instant disclosure concerning so-called main event engine valve motions, i.e., valve motions used during positive power generation, whereas the descriptor “auxiliary” refers to features of the instant disclosure concerning auxiliary engine valve motions, i.e., valve motions used during engine operation other than conventional positive power generation (e.g., compression release braking, bleeder braking, cylinder decompression, brake gas recirculation (BGR), etc.) or in addition to conventional positive power generation (e.g., internal exhaust gas recirculation (IEGR), variable valve actuations (VVA), Miller/Atkinson cycle, swirl control, etc.). An auxiliary motion source 108 is also provided to impart auxiliary motions to the one or more valves 104.
As further shown, an optional automatic lash adjuster 110, 112 may be associated with the main motion load path 106. As used herein, an automatic lash adjuster is “associated” with a motion load path to the extent that it is used to take up lash in the motion load path, and operates either directly within, or parallel to, the motion load path. This is illustrated in FIG. 1 where a first optional automatic lash adjuster 110 is illustrated in-line relative to the main motion load path 106, or a second optional automatic lash adjuster 112 is positioned parallel to the main motion load path 106.
As noted above, compression release engine braking requires opening of an exhaust valve during compression strokes of a cylinder. Given the very high pressures within the cylinder during compression strokes, the force required to open the exhaust valve is relative high. Consequently, the auxiliary motion source 108 and any intervening components along an auxiliary motion load path must be constructed to withstand the comparatively high forces required to open the exhaust valve, i.e., they are commensurately larger thereby increasing manufacturing costs and weight.
Additionally, during valve opening for compression release braking operation, a force or load by the motions imparted by the rocker arm is removed from the automatic lash adjuster. Because this force is absent, the automatic lash adjuster may be free to over-extend or pump-up, i.e., “jacking,” resulting in the plunger excessively protruding from the automatic lash adjuster. As a result, the engine valve may be prevented from fully seating. The partial opening of a valve may ultimately result in poor performance and/or emissions and, in some instances, catastrophic valve-to-piston impact.
Thus, it would be advantageous to provide systems that address these shortcomings of existing systems.